Two basic methods are known for producing furfurylamine; one is by hydrogenation of furfural phenylhydrazone, furfural oxime, furfurylazide or furonitrile, and the other is by direct hydrogenation of furfural in the presence of ammonia. The first method by the hydrogenation of furfural derivatives requires expensive reagents in the preparation of such derivatives, and most of them involve complicated reaction steps to prepare and are difficult to handle. Therefore, the first method is not considered suitable for operation on a commercial scale.
The second method produces furfurylamine by direct hydrogenation of furfural in the presence of ammonia without forming a furfural derivative. In spite of its potential as an economical process for FAM production, the FAM yield obtained by this second method is not high.
For example, U.S. Pat. No. 2,109,159 discloses a process wherein FAM is produced by direct hydrogenation of furfural with a Raney nickel catalyst in the presence of ammonia-saturated cold ethanol. However, this method achieves 80% or less of FAM yield and produces a large amount of difurfurylamine as a by-product. A similar method is described in KOGYO KAGAKU ZASSHI, 53, 24 (1950), but the maximum yield of FAM is 80% and 10% or more of a high-boiling substance is produced as a by-product. According to this literature reference, it is estimated that the reaction proceeds as follows: ##STR1##
In this reaction scheme, furfural readily reacts with ammonia to form furfuramide of m.p. 117.degree. C. Therefore, the hydrogenation reaction using furfural and ammonia requires a sufficient amount of a solvent to dissolve the furfuramide. In addition, according to U.S. Pat. No. 2,112,715, the furfuramide forms an isomer, furfurine (whose structure shown below), at its melting point: ##STR2## and this furfurine does not provide FAM even if it is hydrogenated. It is therefore easily understood that a drop in the selectivity for FAM will occur if the reaction temperature is 117.degree. C. or higher. This suggests the possibility that in the one-step production of 4HFAM, hydrogenation to the tetrahydrofuran ring will not proceed smoothly in such a high temperature range.
The method shown in U.S. Pat. No. 2,112,715 produces FAM by hydrogenating furfuramide per se in an ammonia-water-methanol system, but the yield of FAM obtained is as low as 60%.
In view of the prior art techniques shown above, it is concluded that although hydrogenating furfural in the presence of ammonia would be a very interesting way to produce FAM and/or 4HFAM, the formation of furfuramide as an intermediate makes this method impracticable in industrial application because of the existence of the following disadvantages:
(1) the formation of a high-boiling substance as a by-product;
(2) the need for using a large amount of solvent; and
(3) difficulty in direct formation of 4HFAM due to the upper limit on the reaction temperature.